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Complexometric Determination Essay


Using a Lewis base neutral molecule to donate electron pairs (ligands) to a Lewis acid metal ion center to form a single cluster (complex) ion. When the complex ions forms with a metal ion (chelation) the ligand used is called the (chelating agent). EDTA acts as a great chelating agent due to the Nitrogen and Oxygen donating an electron pair to the metal ion center to form an octahedral complex. The metal ions especially with a +2 charge or higher are the reason for water hardness to form on various objects known as “scum”. Calcium ions are typically the most common contributing factor for water hardness so this experiment uses CaCO3 (Calcium Carbonate) to analyze the hardness of an unknown sample.

A scale of water hardness identifies “soft” water with a value less than 60 ppm (parts per million) and “hard” water with a value more than 200 ppm. 3 mL of ammonia/ammonium chloride buffer (pH 10) is added to the mixture prior to the titration to capture the calcium metal ions so the indicator can work properly. The experiment adds 4 drops of Eriochrome Black T as the indicator to visually see the color change as complexes are formed and the solution undergoes chelation of metal impurities.

The color change from indicator starts as pink and changes to a violet then light blue color to signify the chemical phase changes throughout the reaction until the endpoint. 3 titrations are experimentally conducted to calculate the mean average of the Na2 EDTA for experimental accuracy. The EDTA mean average is then used to calculate the water hardness of an unknown water sample (#97) using 3 more titrations to calculate a mean average of the unknown water sample. An absolute deviation is calculated for each titration experiment to calculate the experimental estimated precision. The final experimental result is then compared to the city of Tempe standard for water hardness and acceptable standards.


(Spurlock, 2014)
(Spurlock, 2014)

“A complex ion is an ion containing a central metal cation bonded to one or more molecules or ions” (Chang, 2013). Just like complex ions, a ligand is a molecule or ion that is bonded to the metal ion in a complex ion (Chang, 2013). A chelating agent is a substance that forms complex ions with metal ions in a solution (Chang, 2013). The process of the chelating agent forming is called chelation. E.D.T.A. (ethylenediaminetetraacetic acid) is a common chelating agent that will be used in this experiment to chelate the metal ions. Tetraamminecopper (II) [Cu(NH3)4]2+ will be the complex ion in this lab experiment. In chapter 11.1-3 the “Kinetic Molecular Theory” is being tested in this experiment (Chang, 2013). Solids are denser than liquids and allow very little empty space to exist between molecules limiting the freedom of motion. The liquids are less dense than solids, held closely together with little space between molecules (less than solids), however, the molecules in liquid do not break away from the attractive forces allowing them to move past each other freely.

Gases are the least dense and have the largest amount of distance between molecules allowing them to move around more freely. According to the theory, the experimental Carbon, Hydrogen, Nitrogen and Oxygen ions are able to quickly attract and find the metal ion in the liquid by donating their electron pair to the metal ion center creating the complex. These complex ions in the experiment use intermolecular and intramolecular forces to break and hold chemical bonds thru the experimental process of chelation to identify the hardness of the unknown sample. After the reaction is complete, when evaporation and or vaporization of the liquid and gas in the molecules is separated the remaining metal impurities known as “scum” are left.

In chapter 4.1 hydration is used to orient the negative poles of the diatomic gases to the positive pole of the Hydrogen and metal impurities in the solution creation the complex cluster. Chapter 4 is also used for titration of redox reactions using a standard solution (Na2 EDTA) to add into another solution of unknown concentration (unknown sample + ammonia/ammonium chloride buffer + Eriochrome Black T) until the equivalence point is reached (has fully reacted) as visually identified by the indicators (Eriochrome Black T) from the color change of pink to violet to blue.


1. “Prepare about 500mL of approximately 0.004M disodium EDTA solution. To prepare your solution, weigh out 0.7-0.8g of Na2EDTA and dissolve in about 500mL deionized water in your plastic bottle. Seal the bottle and shake vigorously for a few minutes to dissolve the salt. 2. Standardize the Na2EDTA solution using a stock calcium ion solution as the primary standard: a. Use a 10-mL transfer pipet to add 10.00 mL of standardized calcium ion stock solution (1.000g CaCO3/L solution) to a 250-mL Erlenmeyer flask. b. Add about 30 ml of deionized water to this titration flask. c. Add a magnetic stir-bar, place on a magnetic stirrer and begin stirring. A piece of white paper under the flask gives good contrast for easier detection of the indicator color change. d. Inside the fume hood, add about 3mL of ammonia/ammonium chloride buffer (pH 10).

The buffer is an inhalation irritant. Stir for 30 seconds. e. Just prior to titrating the flask, add four drops of Eriochrome Black T indicator solution. Continue stirring for another 30 seconds and then titrate this solution with your disodium EDTA solution within 15 minutes. f. Slow down your titration near the endpoint, as the color change takes 3-5 seconds to develop. At the end point, the color changes from pink to violet to blue. If you feel unsure whether you’ve reached your endpoint, read and record the volume delivered and then add another drop of titrant to check for a complete color change. g. Repeat this titration two more times.

Calculate the molarity of your disodium EDTA from each titration. Average your molarities from the three trials and calculate your precision. 3. Choose one prepared unknown water sample as provided. Record the unknown code in your notebook, then titrate this water sample with your standardized disodium EDTA solution: a. Transfer 25.00mL of the prepared water sample to a 250-mL Erlenmeyer flask. b. Add about 20ml of DI water to the titration flask.

c. Add a magnetic stir-bar. Place the flask on a magnetic stirrer and begin stirring. d. Inside the fume hood, add about 3mL of ammonia/ammonium chloride buffer (pH 10). Stir for 30 seconds. e. Just prior to titrating, add four drops of Eriochrome Black T Indicator solution to your flask. Continue stirring for another 30 seconds and then titrate this solution with your standardized disodium EDTA solution within 15 minutes. f. Repeat this titration twice more. Calculate the hardness (mg CaCO3/L) of the prepared water sample from each of your titrations. Calculate your average hardness and your experimental precision from the three trials. 4. Compare your results to the expected range for municipal water hardness. Check your city’s water quality lab website (e.g. http://www.tempe.gov/waterquality/typical_values.htm)” (Complexometric Determination of Water Hardness Lab, n.d.).


Upon adding the preparing the Na2EDTA solution the Na2EDTA solid was quickly dissolved into the DI water to create a clear solution. Later on in the procedure stage of adding the ammonia/ammonium chloride buffer into the 250 mL flask, a visual chemical reaction was observed as the ammonia buffer was mixing into the flask with CaCO3 and the unknown solution in both procedures. Upon adding the Eriochrome Black T indicator the color was visually changed from clear to light pink in both procedures. During titration of both procedures the visual color change was observed from light see-through pink to see-through violet when the process was close to ending, then from see-through violet to see-through light blue signaling then end of the reaction process.

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